1. Field of the Invention
The present invention relates to a driving transmission device in which rotation force of a driving source is transmitted to a driven portion by a belt wound around a driving pulley and a driven pulley, and an image forming apparatus including the driving transmission device.
2. Description of the Related Art
In various known apparatuses including image forming apparatuses such as a copying machine and a printer, a driving transmission device is provided to transmit rotation force of a driving source, such as a motor, to a driven portion.
For example, an image forming apparatus adopts a structure in which driving force of a motor serving as a driving source is transmitted via gears to a driving roller for driving a photoconductive drum and an intermediate transfer belt serving as driven portions. In this structure, however, a rotation transmission error (for example, a mesh transmission error) between a driving gear and a driven gear becomes a vibromotive force and causes vibration. The caused vibration is transmitted to gear support members, such as a shaft, a bearing, and a side plate, and driven components, and this may cause loud noise. When vibration occurs in the image forming apparatus, a striped image or an image having uneven density may be formed, and image degradation, such as streaks and unevenness, may be caused.
Accordingly, there is a driving transmission device in which a belt is wound around a pulley connected to a driving source and a pulley connected to a driven portion. In this driving transmission device, the pulleys and the belt are electrostatically attracted, and transmit driving force of the driving source to the driven portion. For example, in an image forming apparatus described in Japanese Patent Laid-Open No. 8-146783, a driving roller drives an intermediate transfer belt. In this image forming apparatus, electrostatic attraction force is exerted between the driving roller and the intermediate transfer belt by applying a bias to a core metal of the driving roller, so that the driving force of the driving roller is transmitted to the intermediate transfer belt.
When voltage is applied to a conductive portion of a belt and a conductive portion of a pulley to generate an electrostatic force (Johnsen-Rahbek force), current flows through a dielectric layer interposed between the conductive portions, and generates Joule heat.
The length by which the belt is wound on the pulley (winding length) is inversely proportional to the electric resistance value. That is, the resistance value between the pulley and the belt decreases as the area in which the pulley is in contact with the belt increases. For example, the area in which a pulley having a long belt winding length is in contact with the belt with the dielectric layer being disposed between is larger than that of a pulley having a short winding length. For this reason, the total resistance value between the conductive portion of the pulley and the conductive portion of the belt is smaller in the pulley having the long winding length.
For example, when the length by which the belt is wound on the driving pulley is larger than the length by which the belt is wound on the driven pulley, the resistance value between the driving pulley and the belt is larger than the resistance value between the driven pulley and the belt.
In a structure in which the driving pulley, the belt, and the driven pulley are electrically connected in series and the pulleys are attracted to the belt by electrostatic force, the voltage applied to the driving pulley is higher than the voltage applied to the driven pulley owing to the difference in resistance value.
Thus, the attraction force between the driving pulley and the belt is larger than the attraction force between the driven pulley and the belt. For this reason, a slip occurs between the driven pulley and the belt. Therefore, when the driving force is transmitted from the driving pulley to the driven pulley via the belt, it is necessary to set the voltage supplied from a power supply so that a slip does not occur between the driven pulley and the belt.
Since the voltage is applied between the conductive portion of the belt and the conductive portions of the pulleys, the current flowing through the driving pulley and the current flowing through the driven pulley are equal to each other.